Amazon's Kindle Fire "offers a disappointingly poor user experience," according to an expert on usability.

Jakob Nielsen, who has a Ph.D. in human-computer interaction, studied the behavior of four Kindle Fire users. Although that's a tiny sample, Nielsen argues that small, qualitative studies offer more insight than bigger studies focused on metrics.

The Kindle Fire hardware is too heavy, and unpleasant to hold for long periods at a time, Nielsen observed. "Unless you have forearm muscles like Popeye, you can't comfortably sit and read an engaging novel all evening."

Granted, this is true of most 7-inch tablets. And although some lighter tablets are available, such as Samsung's Galaxy Tab 7.0 Plus, PCWorld's Melissa Perenson didn't find the Fire particularly tiresome in her review. Nielsen also derided the Kindle Fire's lack of physical buttons for turning e-book pages, but again, that's true of every tablet on the market (and I don't think it's as frustrating as Nielsen makes it out to be).

The bigger issue, according to Nielsen, is software. The Kindle Fire interface has some trouble spots, like buttons that are too small and screen updates that are too slow. Indeed, sluggish performance was one of the main gripes in PCWorld's Kindle Fire review.

Nielsen also reserved some scorn for apps and websites, as viewed on the Kindle Fire. Mistaken taps were common among users, he noted. Full-sized websites were hard to read and interact with, and mobile-optimized sites are better suited for the 7-inch display.

If Kindle Fire users will kindly put down their pitchforks for a moment, I think there's a good point that Nielsen is trying to make here: you can't shoehorn the interface of a smartphone app or a 10-inch tablet app into a 7-inch display. "A 7-inch tablet is a sufficiently different form factor that it must be treated as a new platform," Nielsen writes.

Until now, app and website developers haven't had any good reason to design for 7-inch displays. The good news is that Amazon's Kindle Fire is becoming a bigger commercial success than any other non-iPad tablet, so we may see more developers catering to 7-inch screens. I don't think users should be satisfied with viewing mobile-optimized websites on a small tablet, especially because so many mobile sites are awful.

And before you get too upset about Nielsen's findings, keep in mind that finding usability weak points is his job. He found plenty of nits to pick with Apple's iPad as well.

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